The 1,2-H shift, 1,2-Me shift, and dimerization reactions of several diaminocarbenes have been studied using density functional theory and ab initio methods. It was observed that the activation energies (E-a) for the dimerization of diaminocarbenes are much smaller than those of 1,2-H shift, 1,2-Me shift and insertion. Dimerization is thus the most likely course of reaction for diaminocarbenes. The activation energies for the dimerization of diaminocarbenes which exhibit 6 pi-electron delocalization are larger than those of the non-aromatic ones. For cyclic diaminocarbenes there is a proportional relation between E-a and the singlet-triplet gap (Delta ES-T), i.e., E-a of dimerization is larger as Delta ES-T increases.